Flow regulator

ABSTRACT

A regulator including a movably, preferably hingedly, mounted member having a distal face, exposed to a reference pressure, and a frontal face, exposed to fluid passing through the regulator. Attached to the member is a structure that variably impedes fluid flowing through the regulator. The amount that this structure impedes the fluid flow varies as a function of the difference between the pressure of fluid on the frontal face of the member and the reference pressure. The impeder may be an integral part of the piston extending into the path of the fluid flowing through the regulator, or it may be a separate structure attached to the member. As the member moves in response to changes in pressure differential across the piston, each movable segment is displaced with respect to its corresponding fixed segment. As the corresponding segments are further displaced with respect to each other, the impedance to flow increases. A restoring force or torque exerts a force on the member so as to tend to lessen the resistance on the fluid flow, and so that when there is no flow through the passageway the resistance is relatively low. The member may be mounted at the top of the regulator in order to make the regulator easy to clean to clean and maintain. A dashpot may be included in the regulator so as to reduce high-frequency oscillations that may occur in a finely tuned regulator. The regulator may include a bypass channel that permits a portion of the flow to bypass the impeder. A variable throttle valve mounted in the bypass channel is used to adjust the effect of the bypass channel.

This application is a divisional of application Ser. No. 08/380,853,filed Jan. 27, 1995, (now issued as U.S. Pat. No. 5,597,011), which is acontinuation-in-part of application Ser. No. 08/141,498, filed Oct. 22,1993, (now issued as U.S. Pat. No. 5,450,873), which is acontinuation-in-part of applications Ser. Nos. 07/965,909, (nowabandoned) and 07/965,907, (now issued as U.S. Pat. No. 5,320,124), bothof which were filed Oct. 23, 1992, and both of which arecontinuations-in-part of applications Ser. Nos. 07/850,767 (now issuedas U.S. Pat. No. 5,251,654), 07/851,017, (now issued as U.S. Pat. No.5,255,710), 07/852,084, (now issued as U.S. Pat. No. 5,255,709) and07/851,016, (now issued as U.S. Pat. No. 5,220,940), all filed Mar. 13,1992, and all of which are continuations-in-part of application Ser. No.07/669,746, filed Mar. 15, 1991, now abandoned, and which is acontinuation-in-part of application Ser. No. 07/405,835, filed Sep. 11,1989, (now issued as U.S. Pat. No. 5,000,221), which is acontinuation-in-part of application Ser. No. 07/178,505, filed Apr. 7,1988, now abandoned. The present application is also acontinuation-in-part of application Ser. No. 08/126,151, filed Sep. 23,1993, (now issued as U.S. Pat. No. 5,456,280). All of these applicationsare hereby incorporated herein by reference.

TECHNICAL FIELD

This invention generally relates to a device for regulating the flow ofa fluid, particularly a gas, through the device.

BACKGROUND ART

In heating, ventilating, and air conditioning (HVAC) systems and houseexhaust systems, air flow is typically controlled using resistors toslow down the flow of air to and from different points in a building.When one resistor is adjusted, the pressure level throughout the systemwill change; any change in the system pressure will affect the flow ofair past every other resistor. Thus, adjusting a resistor at one pointcauses "cross-talk" with resistors at other points.

One of the most complex problems confronted by the HVAC industry iscontrolling air flow through process rooms, such as the clean rooms usedin semiconductor integrated-circuit chip manufacturing. Some air exitsthe process room through process equipment and other work stations withfume hoods. A partial vacuum is usually required in such equipment inorder to ensure that noxious fumes do not leak from the processequipment or fume hoods and thereby endanger personnel working nearby.It is frequently important that a constant partial vacuum be maintainedin, or a constant flow rate be maintained through, the process equipmentin order to minimize defects in the integrated circuit chips beingmanufactured. The fumes from some process equipment may be prone tocondense on regulators mounted between the process equipment and thehouse exhaust system. Regulators used in such situations must befrequently cleaned and maintained.

SUMMARY OF THE INVENTION

The present invention regulates a partial vacuum in a region, such as ina piece of process equipment or a fume hood. The partial vacuum may bewith respect to the environment in which the equipment is located, sothat the region's pressure is between the pressure of the environmentand the pressure of the vacuum source to which the regulator isattached.

The regulator according to the present invention includes a hingedlymounted leaf-piston member having a distal face, exposed to a referencepressure, and a frontal face forming a portion of the conduit wall, themember being hingedly mounted about a pivot point. In a preferredembodiment, the distal face is on top and the frontal face on the bottomof the member. Attached to the member is a gate extending into thepassageway so as to variably impede fluid flowing through the passagewayat a constriction point. Preferably, the gate is in the form of a gratethat is fixedly attached to the member at a point downstream from thepivot point. A fixed grate is mounted immediately adjacent the movablemember's grate, so that the two grates form a plurality of constrictionpoints. The gate's impedance on the fluid flow varies as a function ofthe pressure differential across the member's frontal and distal faces.The gate and the member are mounted so that each moves in a directiontransverse to the direction of the fluid through the constriction point.

The regulator uses a restoring torque on the member around the pivotpoint tending to tend to cause the gate to lessen the impedance on thefluid flow, so that when there is no flow through the passageway theamount that the gate impedes the flow is relatively low. Preferably, therestoring torque is created by the weight of the member upstream of thepivot point. A counterweight may be used in order to create a largeenough restoring torque. The counterweight may be mounted on a rail topermit the restoring torque to be easily modified.

To permit easy removal of the member and the gate, the member preferablyhas a downward-facing groove at the pivot point, so that the member canpivotally rest on a rod fixedly attached to the conduit at the pivotpoint.

To inhibit the member from vibrating, the regulator preferably includesa dashpot formed with a section of the member and a section of theconduit's fixed portion, the two sections being located immediatelyadjacent each other. The movable member's section of the dashpot and thefixed portion of the dashpot define a dashpot volume of fluid, whichvaries as the position of the member varies, but which is alwayssubstantially smaller than the volume of fluid in the conduit betweenthe constriction point and the pivot point. The dashpot is preferablyvented to the reference pressure and to the plenum.

One embodiment of the regulator includes a bypass channel, into whichthe gate does not extend, even when the gate is imparting the greatestimpedance on the flow, so that a portion of the flow through thepassageway bypasses the constriction points defined by the fixed andmovable portions. This embodiment preferably includes a variablethrottle valve for variably restricting the flow bypassing theconstriction points defined by the fixed and movable portions of theimpedance means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of a regulator according to the presentinvention with little or no flow passing through it.

FIG. 2 shows a cross-section of the FIG. 1 regulator with sufficientflow to cause the piston member to pivot downward and increase theimpedance on the flow through the regulator.

FIG. 3 shows a cross-section of an alternative embodiment of theregulator with a bypass throttle valve and with little or no flowpassing through it.

FIG. 4 shows a cross-section of the FIG. 3 embodiment with the bypassthrottle valve adjusted and with a significant amount of flow passingthrough it.

FIG. 5 is a graph representing how the flows through the FIGS. 1 and 3embodiments are regulated.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 shows a preferred embodiment of the present invention. Theregulator shown in FIG. 1 is similar in many respects to the regulatorshown in FIG. 1 of U.S. Pat. No. 5,251,654, (which is referencedhereinabove). Like this earlier regulator, the leaf-piston member 5 ishingedly mounted, and variations in the pressure differential across themember-between the frontal face 52 and distal face 51--will tend torotate the member 5 around the pivot point 84. The distal face 51 ispreferably exposed to ambient pressure, and the regulator maintains aconstant pressure differential between the ambient pressure (or whateverreference pressure is chosen) and the plenum 79, with the plenumpressure being less than the ambient (or reference) pressure.

The FIG. 1 regulator is particularly useful for maintaining a partialvacuum in process equipment, and this is accomplished by attaching theinlet 81 to the process equipment and attaching the outlet 82 of theregulator to a vacuum source. In a typical application, air flows fromthe environment in which the process equipment sits, into and throughthe process equipment, then through the regulator, and finally to thehouse exhaust system. The pressure in the environment, the air of whichis substantially still, is exposed to the member's distal face 51 andthereby serves as the reference pressure.

The member 5 has a gate 96 attached to it. In the FIG. 1 embodiment, thegate 96 is a grate having a pair of arms 967 (only one of which is shownin the cross-section view of FIG. 1) extending downwardly and supportingthe ends of the upstream crosspieces 961, which collectively form themovable grate. Each of the upstream crosspieces 961 has immediatelyadjacent to it a corresponding downstream crosspiece 964, which isconnected to the sidewalls of the conduit. The downstream crosspieces964 form the fixed grate 94, which cooperates with the movable grate 96to form constriction points, through which the flow passes. Thesecrosspieces perform the same function as the airfoil segments shown inFIGS. 5, 6, 7A and 7B of U.S. Pat. No. 5,251,654 (referencedhereinabove), and the grates shown in FIGS. 1 and 5 of U.S. Pat. No.5,320,124 (referenced hereinabove); they create a variable impedance onthe flow though the regulator. (Different arrangements of the grates arepossible; for instance, the fixed grate 94 may be located upstream ofthe movable grate 96, although it will be appreciated that thearrangement shown in FIG. 1, with the fixed grate 96 located downstreamand the movable grate located closer to the pivot point 84, is simplerto construct and disassemble.)

The member 5 is weighted so that, when there is little or no flowthrough the regulator, the gate 96 is pulled all the way up--as shown inFIG. 1--and causes the least impedance. In the preferred embodiment,this is accomplished by a counterweight 90 mounted on a rail 91, alongwhich the counterweight can be moved. The counterweight 90 is mounted sothat it applies a torque on the member 5 around the pivot point 84,opposite of and greater than the torque applied by the gate 96 and therest of the member 5 to the right (as shown in the drawing) of the pivotpoint 84. Thus, the member 5 has a net torque rotating it into itsposition of least impedance (the member's highest position). This torqueis called the restoring torque. The FIG. 1 regulator also includes adashpot 25 located near the pivot point 84; the dashpot 25 is discussedin greater detail below.

FIG. 2 shows the FIG. 1 regulator with sufficient vacuum being appliedto its outlet 82 to cause the member 5 to rotate down, causing theupstream and downstream crosspieces, 961 and 964, to narrow theconstriction points 80 and increase the impedance on the flow throughthe conduit. Because the torque being applied by the counterweight 90 issubstantially constant and because the movable crosspieces 961 movetransversely to the fluid flowing through the constriction points 80,the regulator is able to maintain a constant pressure differentialbetween the reference pressure exerted on the member's distal face 51and the plenum 79--once a sufficient vacuum is applied to the outlet 82to cause the member to rotate downward, and until the vacuum at theoutlet 82 becomes so great that the member 5 cannot rotate down anyfurther. The traverse movement of the movable crosspieces 961 across theconstriction points 80 prevents the movement of the member 5 from beingdirectly affected by the pressure of the vacuum source, so that theregulator may maintain a constant partial vacuum in the plenum 79despite variations in the strength of the vacuum source.

The torque created by the counterweight 90 can be modified by changingthe position of the counterweight 90 along the rail 91. (Of courseseveral counterweights may be used, and each may be mounted on aseparate rail.) The further from the pivot point 848 that thecounterweight 90 is positioned, the greater the torque rotating themember 5 upwardly and the greater the reference-/plenum-pressuredifferential maintained by the regulator. The constant pressuredifferential between the plenum 79 and the member's distal face 51 canbe used to maintain a substantially constant partial vacuum in theprocess equipment to which the inlet 81 is attached, or to maintain asubstantially constant flow rate through the device.

The member 5 rotates up and down in order to compensate for changes inthe vacuum source pressure, as well as changes in the reference pressureagainst distal face 51 and changes in the restrictions on the flow fromthe environment, in which the process equipment sits, to the plenum. (Anexample of a change in the restriction on the flow from the environmentto the plenum would be when the doors of the process equipment areopened and closed: opening the doors decreases the amount of restrictionon the flow from the environment into the equipment and to the plenum;closing the doors increases the restriction on the flow from theenvironment into the equipment and the plenum.) In rotating up and down,the member 5 may become subject to harmonic oscillations, so in apreferred embodiment the regulator includes a dashpot 25 to reducehigh-frequency oscillations. The dashpot 25 may be formed by sections ofthe member 5 and the fixed portion of the conduit. A similar dashpotdesign is shown in the embodiment depicted in FIGS. 3 and 4. As shown inFIG. 4, a section 92 of the member 5 and a section 42 of the fixedportion of the conduit together define a dashpot volume 29. The size ofthis volume 29 is a small fraction of the volume of the plenum 79, belowthe frontal face 52 of the member 5. Because of the small size of thedashpot volume 29, quick large changes in the position of the member5--resulting from quick large changes reference-/plenum-pressuredifferential-would cause enormous changes in the pressure of the gas inthe volume 29. Thus, the dashpot 25 tends to slow any quick largemovements of the member 5. Slow changes in the position of the member5--resulting from slow changes in the reference/plenum-pressuredifferential--are not inhibited by the dashpot 25 because the gas in thedashpot volume 29 can slowly escape from or flow into the dashpot volume29. The dashpot 25 is preferably vented at two locations, one location26 leading into the reference chamber above the member's distal face 52,and the other location 27 leading into the plenum 79. By venting thedashpot volume 29 to the reference pressure and the plenum pressure, theeffect of the dashpot 25 on the regulator's ability to maintain aconstant pressure differential is minimized. (For example, it will beappreciated that if the dashpot volume were exposed to the pressuredownstream of the gate--i.e., the vacuum-source pressure--the positionof the member could be affected directly by the vacuum-source pressure.)The vents 26 and 27 to the dashpot should be very narrow in order togreatly inhibit the flow into and out the dashpot volume 29, withoutpreventing flow into and out of the volume 29.

The two depicted embodiments, one of which is shown in FIGS. 1 and 2,and the other of which is shown in FIGS. 3 and 4, have a design that isadvantageous for use with "wet" process equipment, i.e., equipment whichcreate fumes that may condense in the regulator. By mounting the pistonmember 5 at the top of the regulator, the bottom portion of theregulator may be made solid so that any condensation will be caught atthe bottom of the regulator, from where it can be drained. (If themember 5 were mounted on the bottom of the regulator, condensation couldseep through the gaps between the member 5 and the rest of the conduitwalls.)

The design also allows the easy removal of the member/gate structure.The member 5 has a groove 849 at the location of the pivot point. Thegroove 849 rests on a rod 848 (preferably a Teflon-coated rod to reducefriction) so that the member can rotate around the pivot point and sothat the member/gate structure can be easily removed off of the rod 848.This arrangement permits easy access into the inside of the regulator sothat the regulator's inside and in particular the movable crosspieces961 can be easily cleaned. (Although in the preferred embodiments shownherein, the gate 96 is fixedly attached to, or is an integral portionof, the member, in alternative embodiments the gate may be hingedlyattached to the member. Such an alternative structure makes theregulator somewhat more complicated to disassemble and to clean.)

The embodiment shown in FIGS. 3 and 4 also allows the removal of thefixed crosspieces 964, the pivot rod 848 and the fixed section 42 of thedamper 92 so that these items may also be easily cleaned. The fixedportion of the conduit includes a removable portion 40 that can beunfastened and easily removed from the rest of the regulator. It will beappreciated that this embodiment permits the removal and replacement ofthe entire self-regulating portion of the device, i.e., the member 5,the counterweight 90 and its rail 91, the movable grate 96 (comprisingthe movable cross pieces 961 and the gate arms 967), the fixed grate 94(comprising the fixed crosspieces 964 and the sidewalls 968 to whichthey are attached), the damper 25, and the pivot point 84. Theeasy-removal features of the embodiments shown in FIGS. 1-4 permit theimportant parts regulator to be replaced--while the parts are beingrepaired or cleaned, for instance--without having to shut down theprocess equipment for long periods.

The embodiment shown in FIGS. 3 and 4 includes a bypass channel 60 notfound in the FIG. 1 embodiment. The bypass channel 60 preferablyincludes an adjustable throttle valve 65 to variably constrict thebypass channel 60. The bypass channel 60 permits some of the fluidflowing through the regulator to bypass the constriction points 80formed by the fixed 94 and movable 96 grates. By opening up the bypassthrottle valve 65, the desired plenum pressure (relative the referencepressure) can be reached more quickly. By closing the bypass throttlevalve 65 (as shown in FIG. 3), the regulator of FIGS. 3 and 4 can bemade to function like the non-bypass regulator shown in FIGS. 1 and 2.

FIG. 5 shows how the relative plenum vacuum strength (in, say, negativeinches of water) varies with exhaust vacuum strength (in, say, negativeinches of water, as web) in a regulator with a bypass and in a regulatorwithout a bypass. In both cases, as the exhaust vacuum is turned on andincreases in strength (i.e., as the exhaust pressure drops), the plenumvacuum also increases (i.e., the plenum pressure drops) until the vacuumin the plenum 79 is strong enough to overcome the torque applied by thecounterweight 90 on the member 5, so as to cause the member 5 to rotatedownwardly and narrow the constriction points 80. The narrowing of theconstriction points 80 increases the impedance on the flow between theplenum 79 and the exhaust (at outlet 82). At this point, when theconstriction points begin to narrow, a constant plenum vacuum (comparedwith the reference pressure) is maintained. Because the bypass versionof the regulator has less impedance between the plenum 79 and theexhaust, the desired plenum vacuum is reached more quickly than thenon-bypass version of the regulator.

Eventually, the exhaust vacuum strength will increase so much that themember 5 is pulled down all the way, so that the gate 96 can impede theflow no further. This point is reached in the bypass version of theregulator more quickly (i.e., at a lower exhaust vacuum strength) thanin the non-bypass version of the regulator. Once the member 5 has moveddown as much as possible, the plenum vacuum starts increasing instrength with the exhaust vacuum. Thus, it will be appreciated thatopening the bypass throttle valve 65 causes the region that theregulator maintains a constant pressure--the flat plateau shown on theFIG. 5 graph--to move to lower exhaust vacuums; conversely, closing thebypass throttle valve 65 causes the region where constant plenumpressure is maintained to move to higher exhaust pressures. The throttlevalve 65 may be adjusted accordingly to set the region of the constantplenum vacuum where (along the axis of increasing exhaust vacuumstrength) it will be most beneficial for a given application. As notedabove, the position (or the weight) of the counterweight 90 may beadjusted in order to adjust the desired constant pressure; the greaterthe torque created by the counterweight 90 the greater the pressuredifferential between the plenum 79 and the reference pressure appliedagainst the member's distal face 51.

The new features disclosed herein--in particular, the dashpot 26 and thebypass channel--can be applied to regulators that regulate the flow froma fluid source to an environment such as that shown in FIG. 5 of U.S.Pat. No. 5,255,709 (referenced hereinabove). For instance, by connectingthe right side of the FIG. 3 embodiment to an HVAC or other air source,adapting the member 5 (in particular the damper 25 end of the member) sothat the member/gate structure can rotate up instead of down, andreversing the restoring torque, so as to cause the member 5 to be in itslowest position (and the position causing the least impedance) whenthere is no flow through the regulator. Such a regulator will maintain apressure in its plenum that is a constant amount greater than (asopposed to less than, in the case of the FIG. 3 embodiment) thereference--or environment--pressure. Of course, the fixed section 42 ofthe dashpot 25 would have to be over the dashpot's movable section 92 topermit the member 5 to rotate properly, but the adapted dashpot couldreduce vibration of the member in the manner discussed above. Likewise,the bypass throttle valve 65 could be adjusted to move the region overwhich the adapted regulator maintains a constant pressure.

Although the invention has been described with reference to severalpreferred embodiments, it will be understood by one of ordinary skill inthe art that various modifications can be made without departing fromthe spirit and the scope of the invention, as set forth in the claimshereinbelow.

I claim:
 1. A device for regulating the flow of fluid through apassageway, the device comprising:a conduit having a wall defining thepassageway through the device; a member having a top face, exposed to areference pressure, and a bottom face forming a portion of the conduitwail, the member being hingedly mounted about a pivot point; a gate,attached to the member at a point downstream from the pivot point, thegate extending into the passageway so as to variably impede fluidflowing through the passageway at a constriction point, the gate'simpedance on the fluid flow varying as a function of the pressuredifferential across the member's top and bottom faces, the gate and themember being mounted so that each moves in a direction transverse to thedirection of the fluid through the constriction point; and restoringmeans for exerting a torque on the member around the pivot point so asto tend to cause the gate to lessen the impedance on the fluid flow, andso that when there is no flow through the passageway the amount that thegate impedes the flow is relatively low, the restoring means includingthe weight of the member upstream of the pivot point.
 2. A deviceaccording to claim 1, wherein the member includes a counterweightmovably mounted to the member with respect to the pivot point so as topermit adjustment of the torque exerted on the member.
 3. A deviceaccording to claim 1, wherein the gate includes a movable grate attachedto the member so that it moves with the member, and wherein the conduitincludes a fixed grate fixedly attached to the conduit so that the fixedgrate does not move with respect to the conduit, the fixed and movablegrates being located adjacent each other and defining a plurality ofconstriction points, the size of each constriction point varying as themember moves.
 4. A device according to claim 1, wherein the gate isfixedly attached to the member.
 5. A device according to claim 4,wherein the gate includes a movable grate fixedly attached to the memberso that it moves with the member, and wherein the conduit includes afixed grate fixedly attached to the conduit so that the fixed grate doesnot move with respect to the conduit, the fixed and movable grates beinglocated adjacent each other and defining a plurality of constrictionpoints, the size of each constriction point varying as the member moves.6. A device according to claim 5, wherein the member includes acounterweight movably mounted to the member with respect to the pivotpoint so as to permit adjustment of the torque exerted on the member. 7.A device for regulating the flow of fluid, the device comprising:aconduit through which the fluid flows; a member having a frontal facealong which entire face fluid in the conduit flows, and a distal faceexposed to a reference pressure, the member being hingedly mounted so asto rotate about a pivot point; impedance means for variably impedingflow through the conduit at a constriction point, the size of theconstriction point varying as the member moves, the impedance meansextending only part way into the passageway, so that a portion of theflow through the passageway bypasses the constriction point; andrestoring means for exerting a torque on the member about the pivotpoint so as to tend to lessen the impedance on the fluid flow, and sothat when there is no flow through the conduit the impedance caused bythe impedance means is relatively low.
 8. A device according to claim 7,wherein the impedance means includes movable and fixed portions, thefixed portion being fixedly attached to the conduit so that it does notmove with respect to the conduit, the movable portion being attached tothe member at a point other than the pivot point, so that the movableportion moves as the member moves, the fixed and movable portionsdefining a plurality of constriction points, the size of eachconstriction point varying as the member moves, the impedance meansextending only part way into the passageway, so that a portion of theflow through the passageway bypasses the constriction points defined bythe fixed and movable portions.
 9. A device according to claim 8,wherein the movable portion of the impedance means is attached to themember at a point downstream from the pivot point, and the restoringmeans includes the weight of the member upstream of the pivot point. 10.A device according to claim 9, wherein the member includes acounterweight movably mounted to the member with respect to the pivotpoint so as to permit adjustment of the torque exerted on the member.11. A device for regulating the flow of fluid, the device comprising:aconduit through which the fluid flows, the conduit having a fixedportion; a member having a frontal face, the entire frontal face beingdirectly exposed to fluid in the conduit, and a distal face exposed to areference pressure, the member being hingedly adjacent to the fixedportion so as to rotate about a pivot point; impedance means forvariably impeding flow through the conduit at a constriction point, thesize of the constriction point varying as the member moves, theimpedance means extending only part way into the passageway, so that aportion of the flow through the passageway bypasses the constrictionpoint; and a dashpot formed between a section of the member and anadjacent section of the fixed portion in the conduit so as to define adashpot volume of fluid, the dashpot volume varying as the position ofthe member varies, but always being substantially smaller than thevolume of fluid in the conduit between the constriction point and thepivot point, so that the dashpot inhibits vibration of the member.
 12. Adevice according to claim 11, wherein the dashpot is vented to thereference pressure.
 13. A device according to claim 12, wherein thedashpot is also vented to fluid adjacent the frontal face of the member.14. A device according to claim 11, wherein the impedance means includesmovable and fixed elements, the fixed element being fixedly attached tothe conduit so that it does not move with respect to the conduit, themovable element being attached to the member at a point other than thepivot point, so that the movable element moves as the member moves, thefixed and movable elements defining a plurality of constriction points,the size of each constriction point varying as the member moves.
 15. Adevice according to claim 14 further including a variable throttle valvefor variably restricting the flow bypassing the constriction pointsdefined by the fixed and movable elements of the impedance means.
 16. Adevice according to claim 14 further including restoring means forexerting a torque on the member about the pivot point so as to tend tolessen the impedance on the fluid flow, and so that when there is noflow through the conduit, the impedance caused by the impedance means isrelatively low.
 17. A device according to claim 16, wherein the movableelement of the impedance means is attached to the member at a pointdownstream from the pivot point, and the restoring means includes theweight of the member upstream of the pivot point.
 18. A device accordingto claim 17, wherein the member includes a counterweight movably mountedto the member with respect to the pivot point so as to permit adjustmentof the torque exerted on the member.